Harvard astronomers plan to take advantage of a rare opportunity this spring to see all of the planets in our solar system in a single evening.

The Harvard-Smithsonian Center for Astrophysics (CFA), together with the Harvard Observatory, will be hosting a series of “Star Parties” to enjoy this planetary alignment, which seldom happens in the Northern Hemisphere.

The evening sky in April and May will host the five inner worlds of Mercury, Venus, Mars, Jupiter and Saturn. The early morning sky will be visited by the outer worlds of Uranus, Neptune and Pluto.

The rare alignment would be visible to the naked eye.

The “Star Parties,” which will begin April 20 and run through May 16, are an attempt by the CFA to engage the community in its work.

Assistant Professor of Education Philip M. Sadler, who heads the CFA’s science education department and teaches Astronomy 2, “Celestial Navigation,” is helping the observatory to hold events for adults and children in the Boston area in the coming weeks.

“The planets are on a highway in the sky,” Sadler said. “Something gets backed up and there is a traffic jam, from our perspective.”

CFA spokesperson Christine E. Lafon said the special alignment is a vivid demonstration that the solar system is formed like a disk.

With just a small telescope, she said, the moons of Jupiter and Saturn could be seen in the western sky after sunset.

“Everyone should go out with their evening picnic dinner and check out the sky,” Lafon said.

A complete listing of the “Star Parties” is available at cfa-www.harvard.edu/grandtour/press_release/local_events.html.

On May 5, Mars, Saturn and Venus will group together to form a tiny equilateral triangle in the western sky.

A second unusual alignment will occur on May 10. Mars and Venus will appear to pass so close to one another that, to the naked eye, the two planets will become one.

There will be only three other chances this century to see five planets so tightly grouped: September 2040, July 2060 and November 2100.